1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with an improved nuclear reactor in which the reactor vessel is uniformly supported at its bottom above the reactor containment base mat in a manner which allows the vessel to expand radially, but prevents any lateral motions that might be imposed by a seismic event.
2. Description of the Prior Art
A liquid metal-cooled nuclear reactor (LMR), like other reactors, produces heat by fissioning of nuclear materials which are fabricated into fuel elements and assembled within a nuclear reactor core situated in a reactor vessel. The heat produced by the LMR is used to generate electricity. A typical energy conversion process for the LMR, being similar to that of most commercial nuclear reactors, involves transfer of heat from the reactor core to a primary coolant flow system, therefrom to a secondary coolant flow system and finally into steam from which electricity is generated.
In the LMR, a reactor coolant, such as liquid sodium, is circulated through the primary coolant flow system. A typical primary coolant flow system comprises the reactor core, a heat exchanger and a circulation pump. In a "pool" type system, the nuclear reactor core, the heat exchanger and the circulation pump are located within a large pool of coolant housed within a single vessel, whereas, in a "loop" type system, the heat exchanger and circulation pump are removed from the vessel housing the reactor core and relocated normally in separate vessels.
Generally, there are several heat exchangers and circulation pumps associated with the reactor core. The heat generated by the core is removed by the reactor coolant which flows into the core supporting structure and through the reactor core. The heated reactor coolant then flows through the heat exchangers which transfer the heat to secondary flow systems associated therewith. The cooled coolant exits from the heat exchangers and flows to a circulation pump which again circulates the coolant to the core supporting structure, repeating the described flow cycle.
Although liquid sodium has excellent heattransfer properties and low vapor pressure at temperatures of interest for power generation, and is abundant, commercially available in acceptable purity and relatively inexpensive, making it an attractive medium as a reactor coolant, it does react violently with water which imposes severe problems in the design of sodium-to-water steam boilers. Therefore, reactor safety is a foremost design requirement. Due to the aforementioned characteristics of the liquid metal coolant, sodium, the design must guard against the unlikely happening of loss of coolant around the reactor core. Coolant loss could result from the rupture of the reactor vessel or in the core of a loop type system, rupture of one of the main coolant circulating lines.
Traditionally, most reactor vessels in LMRs are designed to be supported from the top of the vessel. The deck or vessel flange can be cooled, or a support can be provided which allows the vessel support and the reactor foundation to be near the same temperature, so that differential thermal expansions do not occur at the point where the reactor is anchored. Typical examples are the French Phoenix and Super Phoenix designs, and the U.S. FFTF and CRBR designs. These top-supported vessels, being suspended by their shell from the roof or top deck of the reactor, disadvantageously react to seismic loads like pendulums and develop high stresses in the shell near the top of the vessel. Furthermore, they amplify the loads applied to the reactor core within the vessel. This requires more structural material in the core to withstand the loads, resulting in a loss of neutron efficiency. Also, the reactor vessel, being suspended in place, is free to thermally expand downward, away from the support level. This latter characteristic is undesirable for certain accident scenarios in that as the vessel shell heats up, it expands downward moving the core away from the control elements associated therewith, thus leading to an unwanted increase in reactivity.
Over the years, there have also been nuclear reactor designs in which the reactor vessels are bottom supported. Representative of the prior patent art are the bottom supported reactor designs disclosed in U.S. Patents to Wigner et al (U.S. Pat. No. 2,810,689), Zinn (U.S. Pat. No. 2,841,545), Monson (U.S. Pat. No. 2,961,393), Nordheim et al (U.S. Pat. No. 2,990,355), Stoops et al (U.S. Pat. No. 3,007,859), Koutz et al (U.S. Pat. No. 3,072,549 and U.S. Pat. No. 3,120,471), Clifford et al (U.S. Pat. No. 3,257,285), Lagowski (U.S. Pat. No. 3,303,098), Detman et al (U.S. Pat. No. 3,393,127), Greischel et al (U.S. Pat. No. 4,094,737) and Dauvergne (U.S. Pat. No. 4,313,795).
Also, representative of the prior literature art are the bottom supported reactor designs disclosed in an article entitled "A Cold-Bottom Supported Vessel For Sodium-Cooled Reactors" by Didier Costes, in Nuclear Technology, Vol. 67, October 1984, pages 169-176. The Costes article illustrates and describes a variety of bottom supported reactor vessel designs and discusses the possible advantages thereof. The article then proposes a bottom supported vessel design which includes a bottom plate resting on an installation basemat by means of radially flexible supports and a horizontal thin upper flange described as a single-plane bellow extending outside of the upper hoop of the vessel shell and externally clamped to the periphery of the roof deck or slab.
Notwithstanding the potential for some of the bottom supported reactor vessel designs of the abovecited references to overcome many of the pitfalls inherent in prior top suspended reactor vessel designs, it is perceived that a need still remains for an alternative approach to supporting a reactor vessel by its bottom which will substantially overcome the problems experienced with top suspended vessels without creating any significant new ones.